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by Ron Kurtus (updated 18 January 2022)

A galaxy is a collection of a huge number of stars that are rotating around a common center or axis. There is also dust and other material dispersed among the stars.

There are many billions of galaxies in the Universe. Many of the "stars" you see in the night-time sky are really galaxies consisting of a great many actual stars. Our Sun is one star in the Milky Way galaxy.

Questions you may have include:

This lesson will answer those questions.

Stars in galaxies

A typical galaxy consists of between 10 million (107) and 1 trillion (1012) stars slowly rotating around a central axis. It is possible that many of those stars are planetary systems, similar to our solar system. Beside all the stars, there is also interstellar gas and dust rotating around the center.

Top view of galaxy with spiral arms of stars and dust

Top view of galaxy with spiral arms of stars and dust

Dark matter

Some astronomical measurements indicate there is "something else" among the stars and interstellar material. This is called dark matter, because it cannot be seen, but it apparently has mass and does affect other matter and gravitational fields.

Although so-called dark matter appears to account around 90% of the mass of most galaxies, the nature of this material is not well understood.

At the center of some galaxies, there may be super massive black holes. Again, they cannot be seen but are indicated on how they affect nearby stars.

Size and distance

Most galaxies are several thousand to several hundred thousand light years in diameter. A light year is about 5,900,000,000,000 miles (5.9*1012 mi) or 9,500,000,000,000 kilometers (9.5*1012 km).

Edge view of a galaxy 10,000 light years wide

Edge view of a galaxy 10,000 light years wide

Galaxies are usually separated from each other by millions of light years. There are about 125 billion (1.25*1011) different galaxies in the Universe.

Called the "sombrero" galaxy because of its unusal shape

Called the "sombrero" galaxy because of its unusual shape


Many of the "stars" you see in the night sky are actually far-away galaxies. They consist on billions of stars rotating around a center. Gases, dust and dark matter are also part of a galaxy. Some galaxies include one or more black holes near their centers. Galaxies are thousands of light years in diameter and are separated by millions of light years.

You are important in this huge Universe

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